Electric machines having inverters are used in hybrid vehicles, for example, where they are optionally operated in motor or generator operation. In motor operation the electric machine generates additional drive torque, which supports an internal combustion engine, for instance, in an acceleration phase, and in generator operation it generates electric power which is stored in an energy store, such as a battery or a Super-Cab. The type of operation and the power of the electric machine are set using the inverter.
Since comparatively high voltages must be provided in hybrid motor vehicles in order to be able to use the electric machine as a motor to drive the motor vehicle, methods are always provided for protecting the electrical network and the components electrically connected to the electrical network, which prevent permanent damage.
Thus, it is understood that one may disconnect the electric energy store electrically from the network when a malfunction of the electric energy store is detected. If the electric machine is in generator operation at this point, the result is that the network voltage increases greatly, which is able to lead to the destruction of the output stages of the inverter. To prevent this, it is discussed in EP 1 524 761 A1, for example, that one may switch the inverter to short circuit operation, in response to the exceeding of a specifiable hold-off voltage.
German patent document DE 102 21 081 A1 discusses an inverter for an electric machine, which includes a plurality of switching elements which are arranged in a bridge circuit, and which produce a connection between the electrical machine and a battery. The switching elements built into the low-side branch of the inverter are ones that are conductive without a control voltage present, i.e., normally-on switching elements. That ensures that if the supply voltage is absent and the electric machine is rotating, the windings of the electric machine are short circuited and no overvoltages are able to occur.
A method in DE 10 2006 003 254 A1 concerns switching off an electric machine using pulse-controlled inverters in the case of interference, provides for minimizing undesired side effects during the switching off of the electric machine and to maximize the regular machine operation, in that the electric machine is first switched to a disconnect operation in which all the switches of the pulse-controlled inverter are open, and subsequently to a short-circuit mode, in which the switches connected to high potential are open and the switches connected to the low potential are closed.
German patent document DE 10 2007 020 509 A1 discusses a method for fault treatment for electric machines of a hybrid drive, in which it is first detected whether at least one operating parameter of the electrical machine is greater than an associated operating parameter boundary value. If it is detected, in the process, that at least one operating parameter is greater than the associated boundary value, a supply connection of the electrical machine is disconnected for a time interval assigned to the operating parameter and, after the expiration of the time interval, the supply connection is short-circuited to ground.